The number of NTP (Network Time Protocol) servers that can be abused to amplify DDoS attacks has decreased dramatically this year, but the threat remains.

DDoS (distributed denial of service) amplification is an attack technique that involves tricking insecure servers into flooding a target with unrequested data. The method works with several protocols, including DNS (Domain Name System), SNMP (Simple Network Management Protocol) and NTP, that are vulnerable to IP address spoofing by design.

Attackers can send small requests with a forged source IP address to unprotected servers over these protocols to trigger the return of significantly larger responses to the spoofed address, which typically belongs to the intended victim.

In the case of NTP, attackers can use a command called monlist to force servers to respond with the IP addresses of up to the last 600 machines that queried them—a large response compared to the original query—allowing them to amplify the traffic they can generate.

In December, NSFOCUS Information Technology, a network security company based in Beijing, ran an Internet-wide scan and found 432,120 vulnerable NTP servers worldwide. Of those servers, 1,224 were capable of amplifying traffic by a factor greater than 700.

The company repeated its scans in March and May with dramatically different results. In March, NSFOCUS found only 21,156 NTP servers that were still vulnerable to DDoS amplification abuse and their number further decreased to 17,647 in May.

The sharp decline indicates that many network and system administrators have taken steps to restrict access to the monlist feature on their NTP servers, the company said in a report Monday.

The U.S. Computer Emergency Response Team (US-CERT) posted an advisory in January with instructions on how to secure NTP servers by upgrading them to a version newer than 4.2.7 or by disabling monlist manually.

The decline in the number of vulnerable NTP servers observed by NSFOCUS was correlated by researchers from the SANS Internet Storm Center (ISC) using their own data.

“I’d like to suggest that while pundits are citing slow progress for patching Heartbleed, that in actuality, the Heartbleed issue is responsible for the sudden change,” said Kevin Shortt, an incident handler at ISC, Monday in a blog post. “The month of May showed an extensive effort for patching and truing up patch levels because of Heartbleed. This effort likely assisted in the NTP issue being patched along with it.”

However, the danger that insecure NTP servers pose to the Internet is not completely gone. According to NSFOCUS’s data the number of vulnerable servers with an amplification factor of over 700 has actually increased since December, to 2,100.

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